1. Field of the Invention
The present invention relates to methods for forming semiconductor devices. More particularly, the present invention relates to methods for directly writing patterns to create semiconductor devices on semiconductor wafers.
2. Description of the Related Art
Designers and semiconductor device manufacturers constantly strive to develop smaller devices from wafers, recognizing that circuits with smaller features generally produce greater speeds and increased packing density, therefore increased net die per wafer (numbers of usable chips produced from a standard semiconductor wafer). To meet these requirements, semiconductor manufacturers have been forced to build new fabrication lines at the next generation process node (gate length). As the critical dimensions for these devices grow smaller, greater difficulties will be experienced in patterning these features using conventional photolithography.
Conventional photolithography methods used for pattern generation involve exposing a light sensitive photoresist layer to a light source. The light from the source is modulated using a reticle, typically a chrome on glass mask. The patterns formed on the reticle are transferred to the photoresist layer using typically visible or ultraviolet light. The areas so exposed are then developed (for positive photoresist) or, alternatively, the shaded areas are developed for negative type photoresist. The developed regions are then washed away and the remaining photoresist pattern used to provide an etching mask for the substrate.
Unfortunately, the mask or reticle is extremely expensive. Often these costs are disproportionate to the costs involved in other aspects of the chip fabrication. This is particularly the case when the chip is an application specific integrated device with only a small production lot desired. Moreover, often the design of the chip must be modified after testing of chips produced from the first mask. This results in additional expenditures for second, third, and even more sets of masks. Finally, with the reduction in feature sizes, various process limitations in the conventional lithography process have made IC fabrication more difficult.
X-ray and electron beam lithography have been proposed (and adopted in some instances) for imaging very small features. This is because the radiation employed in these techniques has much shorter wavelengths than the ultra-violet radiation employed in conventional photolithography. However, x-ray lithography has found only limited acceptance because of mask, source and resist technology problems. Sources have not been sufficiently bright, and resists have not been adequately sensitive or process-resistant. Further, the x-ray mask is complex to manufacture and does not permit resolution consistent with the theoretical limits set by wavelength. For these reasons, x-ray lithography has not gained widespread acceptance.
Electron-beam lithography (referred to herein as e-beam lithography) involves exposure of a radiation sensitive film to a beam of focused electrons in a vacuum, followed by development of the resist film, and subsequent etching. Thus, e-beam lithography includes the basic steps of conventional lithography, but substitutes a scanning electron beam for an ultraviolet source and reticle. Unfortunately, the imaging step of e-beam lithography is relatively slow. Rather than exposing an entire IC to an image in one shot (as is done in conventional optical lithography), e-beam lithography requires that an electron beam be scanned over the IC wafer surface in a rasterized fashion. To produce a thin line, an e-beam sometimes must be scanned over the line multiple times because the beam size is quite small. This combination of raster scanning and multiple passes requires a long time to produce a pattern image. Thus, fabrication processes employing e-beam lithography have relatively low throughput.
Thus, high costs and maintaining the high precision required for sub-wavelength features are paramount problems.
In view of the above, what is needed is a relatively fast and inexpensive method for transferring images of very thin line width to a wafer. In other words, an effective solution to rising mask costs is needed.